The reason for the failure is line 498 in lib/csv.rb,
in method CSV#inspect. This line reads:

str.map { |s| s.encode("ASCII-8BIT") }.join

The reason for the failure is that currently, filenames on a Mac
are labeled as being in an "encoding" of UTF8-MAC. The label
UTF8-MAC is used to mark the assumption that this string is in a
character normalization form particular to the Mac (mostly NFD,
but not for Korean, and not for CJKV compatibility ideographs,
as far as I understand).

There is in general no knowledge about character normalization with
respect to strings labeled UTF-8 (and even for UTF8-MAC, there is
no guarantee about character normalization at all). In my personal
view, the value of UTF8-MAC is questionable at least at the current
point in time where we do not handle character normalization in
any particular way. But for the current bug, that's actually a side
issue. We might be able to fix this by introducing a (dummy) conversion
from UTF8-MAC to UTF-8, but that won't actually fix the real problem.

The real problem is that the line above ignores that conversion
to ASCII-8BIT only works for US-ASCII characters, but not for
all the other characters that might appear e.g. in a filename.
The easiest fix for this, which is probably what was intended,
is to change the above line to

str.map { |s| s.force_encoding("ASCII-8BIT") }.join

A slightly more "user-friendly" fix is to change this to
something like: